IB/uverbs: Add device memory capabilities reporting

This change allows vendors to report device memory capability
max_dm_size - to user via uverbs command.

Signed-off-by: Ariel Levkovich <lariel@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>